This invention relates to fluid cylinders, either gas or hydraulic, and particularly to repairable type fluid cylinders of the type comprising an assembly in which a tubular body portion is mounted between a pair of end cap members and retained relative thereto by a plurality of threaded tie rod members. The tie rod members are preferably located at a plurality of uniformly spaced locations around the outside of the tubular body portion and may comprise bolt and nut assemblies or merely bolts. In the former instance, the bolts would pass completely through the end caps so that a tightening of the nuts will retain the end caps and body portion. In the latter instance, one end cap would be internally threaded to receive and retain the threaded ends of the bolts, whose heads would engage the axially outer surface of the other end cap. Typically, the axially inner surfaces of the end caps are stepped so as to include a relatively short, axially extending boss portion of circular cross-section which is adapted to fit within one of the cylindrical ends of the tubular body portion to retain it against radial movement. The external periphery of each of the boss portions typically is grooved near its base to receive an elastomeric O-ring which is adapted to contact and form a seal with the inner wall of the tubular body portion placed over it. The O-ring keeps the cylinder from leaking but adds considerably to the expense of making the cylinder since, in addition to the cost of the O-ring, the groove must be machined. Also, a somewhat considerable effort is required to assemble the O-rings to the grooves and to then force the tubular body over the O-rings in order to assemble the tubular body to its end caps. Furthermore, when the tubular body is assembled to the end caps, there is a possibility that the elastomeric O-ring can be damaged. Obviously, it would seem desirable to be able to dispense with the use of an O-ring, but typical cylinders formed, for example, with tubular aluminum body portions and aluminum end caps have been found to be incapable of preventing leakage without the use of elastomeric sealing members such as O-rings. It has also been common to make cylinders where the sealing function is performed by a gasket member. However, in such constructions it is generally necessary to replace the gasket whenever the cylinder assembly is disassembled for repair. Also, the exact length of such cylinder assemblies cannot be assured since the tubular body member tube can be pressed into the gasket to varying degrees depending upon the amount of force applied to the tie bolts and the nature of the gasket material. Furthermore, gasket type seals cannot be satisfactorily used with cylinders which have very thin walls, such as is possible by using stainless steel, since the small dimension of the wall would possibly cut through the gasket material. In addition, where the cylinder is accidently overpressurized, it can be expected that the tie rods will stretch and allow the excess pressure to be released by leakage under the gasket. In such a situation it would be quite likely that the gasket would be forced out of position so that it would not reseal properly when the overpressure conditions was no longer present. In tests of several sizes of cylinders sealed with O-rings, it was found that in every instance that the cylinder was overpressurized sufficiently to cause leakage, the O-ring was extruded out of its groove. Furthermore, when the pressure was released, the O-ring was no longer able to reseal.